The present invention relates to a pressure sensor and a pressure-measuring apparatus. In particular, the present invention concerns a pressure sensor which detects a pressure by detecting a distortion of a thin-film diaphragm due to an introduced pressure, and a manufacturing method of such a pressure sensor. Moreover, it also concerns a pressure-measuring apparatus using such a pressure sensor.
FIG. 1 is a cross-sectional view that shows a structure of a conventional semiconductor pressure sensor of a relative-pressure type (Japanese Patent Application National Publication No. 8-501156). In this semiconductor pressure sensor 1, a doping area 3 that has a conductive type reversed to the substrate 2 is formed on a semiconductor substrate 2, and a diaphragm 5 is formed with a pressure-detecting chamber 4 located in between. Moreover, a pressure-introducing unit 6, which is formed in the semiconductor substrate 2 through antistropic etching, is connected to a pressure-detecting chamber 4 through a pressure-directing path 7.
When, upon application of a pressure P1 (for example, reference pressure) to the outer surface of the diaphragm 5, the resulting pressure P2, introduced through the pressure-introducing unit 6 and the pressure-directing path 7, is applied to the inner face of the diaphragm 5, the diaphragm 5 is distorted by a pressure difference P2xe2x88x92P1. This distortion allows the distance between the diaphragm 5 and the doping area 3 to change, thereby causing the electrostatic capacity between the diaphragm 5 and the doping area 3 to change; thus, based upon this capacity change, it is possible to detect the pressure difference or the pressure.
In the case when such a pressure sensor 1 is used for detecting a varying pressure, as the frequency of pressure variation in the pressure-detecting chamber 4 becomes higher, the shifting speed of a gas comes to have a determined rate, causing the diaphragm to fail to follow it and to distort. The frequency of the applied pressure at this time is referred to as a cut-off frequency.
Depending on applications, the cut-off frequency or the response frequency of the pressure sensor needs to be designed to have a value suitable for the corresponding purpose, and in the manufacturing process, this needs to be controlled to a target value. For example, in the case of the application to a blood pressure meter, etc., since a pump is used to send air to the cuff belt at a constant rate or to reduce the pressure of the cuff belt at a constant rate, the cut-off frequency of the pressure sensor needs to be set higher than the frequency of pulse waves as well as lower than the frequency of pressure noise so as not to allow the pressure sensor to detect the pressure noise from this pump.
The response characteristic or the cut-off frequency of such a pressure sensor is determined by the capacity of the pressure-detecting chamber and the length or the cross-sectional area of the pressure-directing path 7. However, when an attempt is made to control the response characteristic by using the capacity of the pressure-detecting chamber, characters other than the response frequency of the pressure sensor is to change. For example, when the capacity of the pressure-detecting chamber is changed, problems are raised in which there is a change in the area of the diaphragm and there is a change in the gap thickness between the thin-film diaphragm and the doping area, resulting in deviations in the sensitivity of the pressure sensor. Moreover, in a method in which the length of the pressure-directing path is made longer, since the corresponding area is required on the semiconductor substrate, this prevents miniaturization of the pressure sensor. Furthermore, since, in general, the cross-sectional area of the pressure-directing path is as small as several xcexcm2, an attempt to control the response characteristic of the pressure sensor by using the cross-sectional area of the pressure-directing path tends to cause serious deviations in the response characteristic of the sensor unless it is manufactured with very high machining precision.
In one aspect, the present invention has been devised to solve the above-mentioned problems, and its objective is to provide a pressure sensor which can control the response characteristic of the pressure sensor in response to a pressure change without causing any adverse effects on the other properties of the pressure sensor and preventing miniaturization of the pressure sensor.
In another aspect, the present invention relates to various pressure-measuring apparatuses including blood-pressure meters, which are not susceptible to pressure noise derived from pumps, etc.
The pressure sensor in accordance with the present invention, which is provided with a thin-film diaphragm and a cavity that is used for detecting pressure, and formed adjacent to said thin-film diaphragm, which are formed in a sensor main body, is designed to detect a pressure introduced to said pressure detecting cavity based upon the amount of distortion of said thin-film diaphragm, and in this arrangement, a pressure-directing path, which connects a pressure-introducing unit formed in said sensor main body to the pressure-detecting-use cavity so as to direct a pressure from the pressure-introducing unit to the pressure-detecting-use cavity, is formed in the sensor main body, and a space having a cross-sectional area greater than the pressure-directing path is formed in the pressure-directing path. Here, the capacity of this space is preferably designed to be greater than the capacity of the pressure-directing path between the space and the pressure-detecting-use cavity.
In the pressure sensor of the present invention, since the space having a cross-sectional area greater than the pressure-directing path is placed in the pressure-directing path connecting the pressure-introducing unit and the pressure-detecting-use cavity, the pressure (flow of gas), transmitted through the pressure-directing path, is delayed in the space; therefore, even in the event of a temporarily or instantaneous pressure change, the pressure change is absorbed in the space, and prevented from reaching the thin-film diaphragm. Therefore, the formation of the above-mentioned space in the pressure-directing path allows the pressure-directing path to have functions of a low-pass filter to pressure. Further, by adjusting the cross-sectional area and the capacity of the space, the degree of delay of the pressure transmission is controlled so that it becomes possible to control the response frequency and cut-off frequency of the pressure sensor with high precision.
In one embodiment, in the pressure sensor of the present invention, the capacity of the space is determined so as to make the sensor response frequency higher than frequencies in a pressure detection range and lower than frequencies in pressure noise; therefore, it is possible to detect the detection pressure or the varied frequency thereof with high precision without having adverse effects from the pressure noise.
In the case of blood-pressure meters, the frequency of pulse waves to be detected is approximately 15 Hz, while the frequency of pressure noise caused by pumps and valves is approximately 50 Hz; therefore, the response frequency of the pressure sensor is set in a range from not less than 25 Hz that is not less than 1.5 times the maximum frequency of the frequency of the detection pressure or the detection pressure range to not more than 35 Hz. Thus, by setting the sensor response frequency to not less than 1.5 times the maximum frequency of the detection pressure area, it becomes possible to provide a pressure sensor that is particularly suited for the application as blood pressure meters.
In another embodiment, in the pressure sensor in accordance with the present invention, the sensor main body is formed by bonding a first substrate and a second substrate to each other, with the pressure-detecting-use cavity and the space being formed on the surface of the first substrate as recessed portions that are covered with the second substrate; therefore, the pressure-detecting-use cavity and the above-mentioned space are formed by simply processing the surface of the first substrate and then joining the second substrate to the first substrate so as to cover it with the second substrate, thereby making it possible to simplify the manufacturing process.
Furthermore, in this embodiment, the space is formed as a recessed portion deeper than the pressure-detecting-use cavity; thus, it becomes possible to make the cross-sectional area of the above-mentioned space greater than the cross-sectional area of the pressure-detecting-use cavity without widening the width of the above-mentioned space, and consequently to miniaturize the pressure sensor since the occupied area of the above-mentioned space is not widened.
In another embodiment, in the pressure sensor in accordance with the present invention, since the above-mentioned pressure-introducing unit is designed so that it has an opening area on the outer side that is wider than an opening area thereof on the inner side, it becomes possible to easily form the pressure-introducing unit by etching (in particular, antistropic etching on the silicon substrate) the semiconductor substrate constituting the sensor main body.
In another embodiment, in the pressure sensor of the present invention, the above-mentioned space may be placed at a position that is in contact with said pressure-introducing unit or in the middle of said pressure-directing path. In either of the cases, since the above-mentioned space is not adjacent to the pressure-detecting-use cavity, it is possible to make the characteristics of the pressure sensor less susceptible to influences from the space. Further, in the case when the above-mentioned space is placed at the position in contact with the pressure-introducing unit, it is only necessary to place the pressure-directing path only on one-side of the above-mentioned space; thus, it becomes possible to miniaturize the pressure sensor. Moreover, when the above-mentioned space is placed in the middle of the pressure-directing path as described in the latter case, even upon invasion of dusts, etc. through the pressure-introducing unit, these are caught in the space and less likely to reach the pressure-detecting-use cavity, thereby making it possible to reduce the adverse effects of dusts, etc. on the sensor characteristics. Here, in the case when the above-mentioned space is placed at a position adjacent to the pressure-introducing unit, an arrangement is made so that the pressure-introducing unit and the above-mentioned space are aligned in the thickness direction of the sensor main body; thus, it is possible to reduce the installation spaces of the two members, and consequently to miniaturize the pressure sensor.
In another embodiment, in the pressure sensor of the present invention, a plurality of pressure-detecting-use cavities connected by a pressure-directing path are placed, and the space is formed in a pressure-directing path that reaches from the pressure-introducing unit to the first pressure-detecting-use cavity; therefore, it becomes possible to reduce the portions in which the above-mentioned spaces are formed, and consequently to miniaturize the pressure sensor.
In another embodiment, in the pressure sensor of the present invention, a plurality of pressure-detecting-use cavities connected by a pressure-directing path are placed, and the above-mentioned space is formed at a position before a portion at which the pressure-directing path extended from said pressure-introducing unit is branched to the respective pressure-detecting-use cavities; therefore, it becomes possible to reduce the portions in which the above-mentioned spaces are formed, and consequently to miniaturize the pressure sensor.
The pressure-measuring apparatus of the present invention is provided with a pressure sensor disclosed in the present invention; and a calculation circuit for calculating a pressure value from electric signals that are outputted in accordance with the pressure detected by the pressure sensor.
In one embodiment of the pressure-measuring apparatus of the present invention, since it is possible to eliminate pressure noise that is caused by a pump and a valve, it becomes possible to positively hold pulse waves that form a pressure to be detected, and consequently to achieve a blood pressure meter that has a high S/N ratio. Moreover, since it is not necessary to provide circuits, etc. that electrically eliminate noises, it is possible to miniaturize the pressure sensor, and consequently to achieve low prices. In particular, in the case of the application to a pressure sensor used for fingers, it is possible to miniaturize the blood pressure meter, and consequently to provide a convenient meter in use.
Additionally, the constituent elements as described above can be desirably combined as widely as possible.